New to the Forum

Hi my name is Kevin, and I'm a newtubeaholic. I just built a ST120 amp from Bob Latino, and I'm enjoying it immensely, when I'm not breaking it. So far I had one of the KT88's that wouldn't bias (Bob replaced it), I broke a 12AT7, and I think I blew out the tube rectifier also. Luckily I bought a SS rectifier from Bob also, so I'm playing the tunes. I can't figure out how to post pics on this forum. Can someone help me out? I am pretty active on the Klipsch Community forum, and I posted a couple of pics there. Question: I have a little hum at idle, more prevelant in the left channel. Not noticable when music is playing. Is it anything to be concerned about? My speakers are very efficient,(very modified Khorns-105db/watt) and tend to bring out every little thing. I have some Mullard tubes ordered for the VTA board, plus a Weber rectifier. I'm thinking of ordering some re-issue Genelex KT-88's also. I'm hoping that new tubes will lessen the hum at idle. I also noticed the hum is lessened if I put my finger on top of the 12AT7 tubes, especially the one in the center. Is this normal or do I have a ground issue, and will tube dampers make a difference? Thanks for your patience with me! Kevin

Glad that the new KT88 tube got you up and running again. The center 12AT7 is the tube most likely to cause hum. This tube has two triode sections and is the initial voltage amplifier for BOTH channels. The triode section for the left channel on this tube could cause a hum issue that could affect both channels but may be more prevalent on the left side. One issue that could cause the hum to be more noticeable is the fact that you have Klipsch speakers with a high 105 dB efficiency. A higher efficiency speaker will tend to magnify any hum issue the amp may have. The ST-120 also has a little more gain than the average tube amp. I will send you some information off the board to drop the gain of the ST-120. This might be advisable since your speakers are very efficient.

To check for hum on the ST-120 (or the ST-70 for that matter) disconnect all associated equipment from the ST-120 and then insert two RCA interconnects into the two input jacks. Now > short between the center pin and the outer shield on THE OTHER SIDE of the two interconnects with a pair of jumpers or the metal end of a screwdriver. If the hum is still there then it is an issue IN THE AMP. If the hum goes away then your associated equipment is the cause of the hum.

This forum is supposed to be able to host photos but I couldn't make it work either. What you (and anyone else who wants to post a photo on this forum) should do is join a FREE web photo hosting web site like Photobucket. It doesn't cost anything. All the photos I have posted here on the Forum have been on Photobucket > Photobucket

Thanks Bob, I tried Photobucket and it seemed to work. Maybe you could check out my post and see if you can see the pics. I got your e-mail about changing the resistors. I understand what you are saying about changing the gain profile, but I don't think I'll change anything right now since I'm planning to replace my preamp with a tube preamp, and I am in the process of making some more modifications on my speakers, which will obviously impact on the overall sound of the system. But I'm going to keep the modification in mind in case I need to use it. Thanks, Kevin

I painted the amp bells gold to match the Shanling CD player, and hopefully the next preamp I get. I can always paint the turntable gold, which was my original intention when I built it. But I'm looking to replace it soon also.

Tube Nube wrote:YOu built the turntable? A DIY turntable? oh,Kev, do tell!

OK, I didn't build the turntable, what I did was take my old JVC QL50 turntable and build a different plinth for it. Of course the QL50 model didn't come with a tonearm, so it had and aftermerket arm installed, which is a Signet. That made it easier to build. Anyway, I'm getting ready to build another from a JVC QL-7 turntable. This one will be solid oak, to match my speakers.

kevinmi wrote:Oh by the way, I got my Mullard tubes in the mail today (3-4024's) so I'm off to the Man Cave to do a little tube rolling!

Hi Kevin,

Those Mullard CV-4024 tubes are probably the best 12AT7 tubes you can use in the VTA ST-70 or VTA ST-120. Telefunken tubes are also very good but are harder to find than the Mullard's. One curious thing about Mullard small signal tubes (12AX7, 12AT7, 12AU7 etc.) is that they exhibit what has been called the "Mullard Flash". When these Mullard tubes are COLD the filament will flash brightly for a second - especially the center 12AT7 on the VTA driver board. When the tubes are already warm they won't flash. Some users think something is wrong with their Mullard tubes but they are fine and it won't harm them or cut down on their life span.

Thanks for some details of the turntable project, Kevin. I'm interested to hear that you did this.

In the last year I've made some changes to my Linn turntable. I don't know what possessed me to do it myself. I was probably emboldened by the completely positive experience I had with my first upgrade project -- my ST 70.

I think Linn tries to scare people by suggesting it's a delicate, finicky business messing with their decks -- not for the faint of heart -- probably ought to be a trained professional, blah blah.

Of course, I might have the thing set up all wrong, but it sounds good to my ears.